Leaving aside the dubious origins of the idea, many Japanese (along with others in East Asia) believe your personality is intrinsically linked to your blood type, or ketsueki-gata.
Even more than horoscopes in the West, blood type captures the imagination of a large portion of the Japanese population who catapult books on the subject into best sellers. These books provide extensive information on the best and worse character traits of people with each type, outline the best diet for each type, and predict compatibility with other blood types.
Blood type horoscopes are listed daily in Japanese newspapers and announced on morning TV. And Japanese baseball cards list the player's blood type.
The interest in ketsueki-gata isn't limited to popular culture, either. There are, in fact, companies that base their hiring on (blood) types that would work well with others or be stable employees. At one point Mitsubishi Electronics announced the formation of a team comprised solely of blood type AB workers because of "their ability to make plans".
Politicians and other public figures have been known to use their blood type as an explanation or excuse for their behavior. Ryu Matsumoto, for example, who was forced to resign as reconstruction minister in 2011 after making remarks that quake victims found offensive, blamed his conduct on his blood type.
Blood types discussion frequently comes up in casual after-hours conversation, so don't be surprised if you're asked, "What's your type?" And don't be surprised if the Japanese are stunned if you don't know yours.
In addition to checking up on your "type," you might want to have some idea of the assumptions the Japanese may be drawing based on your hematology.
TYPE A: Organized, calm, sensitive, introverted, responsible, timely, perfectionists and precise, but can be self conscious, overly anxious, uptight, and obsessive.
TYPE B: Easy going, cheerful, creative, free-spirited, loving, unique, somewhat eccentric, passionate, but can only focus on what they are interested in and can be selfish, forgetful, and irresponsible.
TYPE O: Curious, personable, generous, outgoing, social, ambitious, good leaders, can get along with anyone, but don't like tedious details, and can be stubborn, vain, jealous, insensitive and egotistical.
TYPE AB: Rational, arty, empathetic, bookworms, trustworthy, and very organized, but can be detached, mysterious, judgmental, indecisive, unpredictable, and unforgiving.
Of course, many Japanese also disregard this as superstition. What do you think now that you know what is associated with yours?